The present invention relates to the field of fused evanescent wave couplers, and more specifically to polarization independent optical fiber couplers and a method for fabricating them.
Coupling in fused single-mode fiber couplers is wavelength dependent, and this property can be used to realize optical fiber wavelength division multiplexing (WDM) components. WDM couplers operating at transmission wavelengths of 1.3 .mu.m and 1.55 .mu.m, corresponding to the fiber attenuation minima and to emission wavelengths of readily available laser transmitters, are commercially available. These devices exhibit typical losses of less than 0.5 dB with channel cross-talk of less than -16 dB. Cross-talk is limited by the unequal coupling coefficients for the two orthogonal polarization states which are possible at the coupler input.
For a long-haul fiber bidirectional or duplex transmission system, link length between repeaters is limited by the fiber attenuation at 1.3 .mu.m, which is typically 0.4 dB/km. If the system were designed so that the laser transmitters operate near the minimum attenuation wavelength of 1.55 .mu.m, where the loss is typically 0.2 dB/km, the link length could be increased significantly.
To take advantage of the minimum attenuation near 1.55 .mu.m, and to allow for the potential use of erbium doped fiber optical amplifiers presently under development, the WDM channel separation should be less than 50 nm. This amount of wavelength separation is achieved in fiber couplers if they are strongly over-coupled, that is, their coupling lengths are long and they exhibit many power transfer cycles along their length. For such large coupling lengths, the polarization birefringence of the coupler becomes increasingly significant, and it is the birefringence that can limit the attainment of low values for the channel cross-talk.
Demonstrations of WDM couplers with channel spacings as small as 100 nm in the wavelength region near 1.3 .mu.m have been reported. For these devices, the number of transfer cycles is 10, and the worst-case cross-talk value was determined to be -18 dB, which again is limited by the polarization birefringence of the coupler.
A typical coupler consists of a pair of single-mode fibers which have been fused together over a specified length. Coupling theory proposes that an optical signal launched into one of the input fibers escapes from the fiber core and excites the lowest order symmetric and antisymmetric modes of the waveguide formed by the fused fiber section and surrounding medium, such as air. These modes interfere along the length of the fused fibers and produce a coupler output field whose amplitude depends on the difference between the propagation constant of the two modes and the length over which the fibers are coupled. Because the propagation constants and their differences are not equal for the x and y polarizations due to the cross-sectional asymmetry, the coupler exhibits birefringence. Therefore, in general, the fraction of coupled power will not be the same for the two polarizations. This also means that for unpolarized light, which characterizes laser signals propagated over an appreciable length in fiber, an efficient low cross-talk WDM coupler, which requires 0 and 100% coupling at the two transmission wavelengths, cannot be realized unless special designs are implemented which negate the effect of the coupler birefringence.
Therefore, a need exists for a fiber optic coupler which provides 0 and 100% coupling at two transmission wavelengths where the effects of coupler birefringence are negated.